This invention concerns improvements in or relating to breathing apparatus.
There are two main types of breathing apparatus in use at the present time. One type consists of an air or other respirable gas supply from a cylinder controlled by a demand valve, operated by the wearer (commonly called open circuit type). In this type the full volume of each breath is taken from the cylinder. The British Standard test for breathing apparatus specifies a minute volume of breathing of 40 liters a minute (BS 4667 1974). This has been shown to accurately reflect that actually needed in practice. For practical purposes the largest size oxygen cylinder a man can carry in a breathing apparatus has 2000 liters of air in it and this charged weights some 15 kilograms. It is seen from these figures that the maximum duration of this type of breathing apparatus is 60 minutes at this flow.
When the requirement for a breathing apparatus is for a longer duration of up to 4 hours, a second type of breathing apparatus is used. This type of apparatus, commonly referred to as the closed circuit type, consists of an oxygen cylinder or other oxygen supply feeding into a recirculating system. The circuit of this system includes a reservoir bag and a carbon dioxide absorber, together with non-return and relief valves. This system is well known and has the advantage that the oxygen supply from the cylinder can now be at the rate absorbed by the wearer's lungs and is approximately 1-3 liters per minute depending on activity. This means that the oxygen used for a 4 hour duration is now only 720 liters maximum (at 3 liters per minute) giving a large weight saving. The disadvantage of all known systems of this type, however, is that the absorption of carbon dioxide by an absorbent material, usually soda lime in a cannister, involves an exothermic reaction. The amount of heat produced by this reaction is of the order of 1000 calories per minute for the figures given earlier, and thus the medium breathed by the wearer of the apparatus is correspondingly heated.
As breathing apparatus is commonly used in hot environments such as fires or mines, this heat is not easily dissipated and consequently places a physiological strain on the wearer. The British Standard test mentioned earlier specifies an inhaled oxygen temperature of less than 40.degree. C. in an ambient of 30.degree. C. 85-90% relative humidity. It has been shown that if this inhaled oxygen temperature could be reduced below the body temperature of 37.degree. C., great benefit would result for the wearer, as he would now pass heat to the apparatus instead of taking heat from it. Many methods to achieve this have been tried, for example, coolers using latent heat of fusion of ice or other substances, but these to be effective are too heavy for the wearer. Evaporation of water has been also tried but as the relative humidity of the environment is high, elevated flows of oxygen are needed in combination with this, again producing too heavy an apparatus.